I installed and used Redhat9, Debian, W2K on my laptop. Each of them on one partition.
All I used DHCP. Debian and W2K can access to Internet and work very well but I have a problem with Redhat9 as the following:
Redhat in my laptop detected exactly IP address of gateway and DNS server.
I use DHCP, system assigned IP address 192.16.126.242 to my Laptop.
IP address of gateway 192.16.126.19,
IP address of DNS server 192.16.124.50.

I can ping to gateway computer and all computers that have IP address 192.16.126.*, but I can not ping to DNS server.
I checked /etc/resolv.conf file and it pointed to correct DNS server.
DNS server is no problem because when I switch to W2K or Debian then they run very well.

Can you help me to solve this problem ?

Have you checked redhat-config-firewall to make sure that you are allowing the traffic through. DNS uses UDP by default and for query packets that are less than 512 bytes.

Try to flush your firewall and see if the problem goes away:
iptables -F

I do not install firewall in my Redhat9 Laptop so I cannot check
Redhat-config-firewall as you suggest because it doest exist.
I do not know where is the packet contain Redhat-config-firewall
I only install Redhat and use it as environment to run network simulation (NS-2) and WWW. Now, I can not reach to DNS server so I can not use web when I am working in Redhat environment. It is not convenient!!

Can you suggest me how to do to solve this problem.

1) What is the subnet mask of the linux box?

2) What is your gateway (another computer/broadband router/etc)?

3) What error do you get (timeout/ect) when you try to ping the DNS server?

Dear DirtDart

Subnet mask is 255.255.255.0
Gateway is 192.16.126.19
I can ping to gateway 192.16.126.19 and all computer in subnet. However I can not ping to DNS server 192.16.124.50. When I ping to DNS server, it no answer for long time.

I am Looking forward the answer from you

Now, my Redhat9 can run very well without any problem.
I remove IP address that Redhat assign automaticly in order to Redhat can provide other IP but it assign the same.
After that I bring my laptop to other room of student house. That room belong to other subnet. Of course Redhat must auto detect and assign other Ip address to my Laptop.
From at that time, Redhat can ping DNS server (DNS server is the same) and access to Internet.
I bring my labtop back at my room. This time Redhat assign other IP address that is different with the first time and Redhat can run OK at my room. Now, I can access to Intrenet without any problem.
I fell very happy now.